1. Technical Field
The present invention relates to battery chargers in general, and in particular to a method for reducing charging time of a battery having a charging rate greater than the capacity of a battery charger.
2. Description of Related Art
Lithium ion rechargeable batteries can be found in many portable electronic devices such as notebook personal computers (PCs). Although lithium ion rechargeable batteries have high energy density, the charging time of lithium ion rechargeable batteries tend to be long. In recent years, a fast charge type lithium ion rechargeable battery that employs nano-sized particles as negative-electrode material has been developed and begun to appear on the market. A normal charge type lithium ion rechargeable battery has a charging rate of about 0.7 C. In contrast, a fast charge type lithium ion rechargeable battery has a charging rate of about 2 C. As a result, the charging time for a fast charge type lithium ion rechargeable battery is much shorter than the charging time for a normal charge type lithium ion rechargeable battery.
When a fast charge type lithium ion rechargeable battery is charged by a battery charger having a capacity capable of charging normal charge type lithium ion rechargeable batteries, the fast charge type lithium ion rechargeable battery cannot perform up to its full capability because of the insufficient charging capacity. For example, a fast charge type lithium ion rechargeable battery can be charged up to 80% of its full charge capacity within 30 minutes by a battery charger having a capacity suitable for its charging rate. However, when the fast charge type lithium ion rechargeable battery is charged by a battery charger adapted for normal charge type lithium ion rechargeable batteries, the fast charge type lithium ion rechargeable battery may take 75 minutes to be charged to the same capacity.
Typically, it is desirable for a notebook PC to be charged to as much capacity as possible within a short time after usage in a mobile environment so that it can be prepared for later mobile use. It is also expected that the fast charge type lithium ion rechargeable battery would be gradually employed in future notebook PCs. However, when the capacity of a battery charger mounted within a notebook PC is increased in order to be able to handle the fast charge type lithium ion rechargeable battery, the battery charger becomes bulky, which may cause problems such as reducing inner space of the notebook PC, increasing heat generation and increasing cost.